70 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



SECTION 4. Tlie Nerve Tube. 



This is colored violet in the sections, like the ectoderm, as it is ecto- 

 dermal in origin, although very young stolons must be studied to show 

 this. In all the older stolons it is distinct from the ectoderm from end 

 to end, and appears in all the sections, as it is shown in Plate XXXIV, 

 Fig. 1, at Z, as a closed tube, with a lumen, in contact with the inner 

 surface of the ectoderm, on the middle line of the upper surface of the 

 stolon. Even in a stolon as young as the one shown in Plate XXI, 

 sections throw no light on its origin, for Fig. 7 shows that it has the 

 structure that it has in the figure just referred to. In the stolon, a little 

 older than Plate XX, Fig. 7, w r hich is shown in transverse section in Figs. 

 1, 2, 3 and 4, I found the clearest evidence of its ectodermal origin. In 

 the more proximal sections 1, 2 and 3, the ectoderm covers only the 

 bottom and sides of the stolon, but the tip of the distal end, Fig. 4, is 

 completely shut in, although its ectoderm, a', has not yet separated from 

 the ectoderm, a, of the solitary embryo, and at the point where the two 

 layers are continuous, the abundance of nuclear figures shows active cell 

 multiplication, and a number of the cells have pushed in to the cavity of 

 the stolon, where they form a projecting knob, I, the rudimentary nerve 

 tube. At this early stage it is solid, and its lumen, which appears 

 later, never has any communication with the exterior. At least I have 

 found no opening, although Salensky, who has recently shown (17) that 

 the nerve tube of the ascidiozooid of Pyrosoma is ectodermal, finds a dis- 

 tinct neural invagination. A longitudinal section of the somewhat older 

 stolon of Plate XLI, Fig. 3, is shown in Plate XVI, Fig. 5. Here the 

 nerve-rudiment, I, stretches for some distance along the middle line of 

 the stolon, and at its distal end the cells are arranged around its axis, 

 although the lumen has not yet made its appearance. Plate XXXV 

 shows that the stolon is about as far away from the ganglion of the 

 solitary embryo as it could well be, and it is difficult to believe that 

 there is any phylogenetic connection between the nerve tube of the 

 stolon and the ganglion of the embryo. It is of course possible that 

 ontogenetically the two structures may be common descendants of the 

 same cells, but there is no indication of any such ontogenetic relation- 

 ship. Uljanin believes (7) that the budding of salpa is to be traced back, 

 phylogenetically, to fission in a double embryo. If this has been its 

 history we certainly should expect to find corresponding structures 

 arising in positions which are more consistent with their supposed 



